A prior art type of this semiconductor electret capacitor microphone is roughly comprised of a semiconductor chip, a vibration membrane opposite a fixed electrode formed on the semiconductor chip, and a case for enclosing the semiconductor chip and the vibration membrane. On the semiconductor chip, a necessary electronic circuit is formed and also the fixed electrode is formed on an insulating layer formed on the surface thereof. This semiconductor chip has a spacer formed thereon.
The vibration membrane is attached to a ring-shaped vibration-membrane ring and assembled in such a state that it is spaced through the spacer from the fixed electrode by a predetermined distance. On a face of this vibration membrane opposite the fixed electrode is formed an electret layer, between which and the fixed electrode is constituted a capacitor.
The above-mentioned prior art semiconductor electret capacitor microphone, however, has the following problems.
That is, since as shown in FIG. 12 a fixed electrode 110 formed on a semiconductor chip is connected to an input electrode thereof (gate of an FET shown in FIG. 12) and a vibration membrane 200 is connected to the ground, respectively, a parasitic capacitance C of a few tens of pico-farads is generated below the fixed electrode 110. This parasitic capacitance C is larger than a capacitance (2 pF) of the capacitor to thus generate large noise in an output, resulting in a problem of a droop of 20 dB in sensitivity as compared to the prior art semiconductor electret capacitor microphone.
In view of the above, it is an object of the invention to provide a semiconductor electret capacitor microphone that can greatly improve a noise level.